.\"	$OpenBSD: EVP_DigestVerifyInit.3,v 1.15 2024/07/21 08:36:43 tb Exp $
.\"	OpenSSL fb552ac6 Sep 30 23:43:01 2009 +0000
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.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
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.Dd $Mdocdate: July 21 2024 $
.Dt EVP_DIGESTVERIFYINIT 3
.Os
.Sh NAME
.Nm EVP_DigestVerifyInit ,
.Nm EVP_DigestVerifyUpdate ,
.Nm EVP_DigestVerifyFinal ,
.Nm EVP_DigestVerify
.Nd EVP signature verification functions
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_DigestVerifyInit
.Fa "EVP_MD_CTX *ctx"
.Fa "EVP_PKEY_CTX **pctx"
.Fa "const EVP_MD *type"
.Fa "ENGINE *engine"
.Fa "EVP_PKEY *pkey"
.Fc
.Ft int
.Fo EVP_DigestVerifyUpdate
.Fa "EVP_MD_CTX *ctx"
.Fa "const void *d"
.Fa "size_t cnt"
.Fc
.Ft int
.Fo EVP_DigestVerifyFinal
.Fa "EVP_MD_CTX *ctx"
.Fa "const unsigned char *sig"
.Fa "size_t siglen"
.Fc
.Ft int
.Fo EVP_DigestVerify
.Fa "EVP_MD_CTX *ctx"
.Fa "const unsigned char *sig"
.Fa "size_t siglen"
.Fa "const unsigned char *tbs"
.Fa "size_t *tbslen"
.Fc
.Sh DESCRIPTION
The EVP signature routines are a high-level interface to digital
signatures.
.Pp
.Fn EVP_DigestVerifyInit
sets up verification context
.Fa ctx
to use digest
.Fa type
and public key
.Fa pkey .
.Fa ctx
must be initialized with
.Xr EVP_MD_CTX_init 3
before calling this function.
If
.Fa pctx
is not
.Dv NULL ,
the
.Vt EVP_PKEY_CTX
of the verification operation will be written to
.Pf * Fa pctx :
this can be used to set alternative verification options.
Any existing value in
.Pf * .Fa pctx
is overwritten.
The
.Vt EVP_PKEY_CTX
value returned must not be freed directly by the application.
It will be freed automatically when the
.Vt EVP_MD_CTX
is freed.
The
.Fa ENGINE *engine
argument is always ignored and passing
.Dv NULL
is recommended.
.Pp
.Fn EVP_DigestVerifyUpdate
hashes
.Fa cnt
bytes of data at
.Fa d
into the verification context
.Fa ctx .
This function can be called several times on the same
.Fa ctx
to include additional data.
This function is currently implemented using a macro.
.Pp
.Fn EVP_DigestVerifyFinal
verifies the data in
.Fa ctx
against the signature in
.Fa sig
of length
.Fa siglen .
.Pp
.Fn EVP_DigestVerify
verifies
.Fa tbslen
bytes at
.Fa tbs
against the signature in
.Fa sig
of length
.Fa siglen .
.Fn EVP_DigestVerify
is a one shot operation which verifies a single block of data
in one function call.
For algorithms that support streaming it is equivalent to calling
.Fn EVP_DigestVerifyUpdate
and
.Fn EVP_DigestVerifyFinal .
.\" For algorithms which do not support streaming
.\" (e.g. PureEdDSA)
.\" it is the only way to verify data.
.Pp
The EVP interface to digital signatures should almost always be
used in preference to the low level interfaces.
This is because the code then becomes transparent to the algorithm used
and much more flexible.
.Pp
The call to
.Fn EVP_DigestVerifyFinal
internally finalizes a copy of the digest context.
This means that
.Xr EVP_VerifyUpdate 3
and
.Xr EVP_VerifyFinal 3
can be called later to digest and verify additional data.
.Pp
Since only a copy of the digest context is ever finalized, the context
must be cleaned up after use by calling
.Xr EVP_MD_CTX_free 3
or a memory leak will occur.
.Sh RETURN VALUES
.Fn EVP_DigestVerifyInit
and
.Fn EVP_DigestVerifyUpdate
return 1 for success and 0 for failure.
.Pp
.Fn EVP_DigestVerifyFinal
and
.Fn EVP_DigestVerify
return 1 for success; any other value indicates failure.
A return value of 0 indicates that the signature did not verify
successfully (that is, the signature did not match the original
data or the signature had an invalid form), while other values
indicate a more serious error (and sometimes also indicate an invalid
signature form).
.Pp
The error codes can be obtained from
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr evp 3 ,
.Xr EVP_DigestInit 3 ,
.Xr EVP_DigestSignInit 3 ,
.Xr EVP_PKEY_meth_set_verifyctx 3
.Sh HISTORY
.Fn EVP_DigestVerifyInit ,
.Fn EVP_DigestVerifyUpdate ,
and
.Fn EVP_DigestVerifyFinal
first appeared in OpenSSL 1.0.0 and have been available since
.Ox 4.9 .
.Pp
.Fn EVP_DigestVerify
first appeared in OpenSSL 1.1.1 and has been available since
.Ox 7.0 .
